From the early stage of providing voice-oriented services, a mobile communication system has evolved into a high-speed and high-quality wireless packet data communication system to provide data and multimedia services. Various mobile communication standards such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution-Advanced (LTE-A), High Rate Packet Data (HRPD) of the 3rd Generation Partnership Project-2 (3GPP2), and IEEE 802.16 have recently been developed to support high-speed and high-quality wireless packet data communication services. In particular, the LTE system, which is a system developed to efficiently support high speed wireless packet data transmission, maximizes wireless system capacity by using various wireless access technologies. The LTE-A system is an advanced wireless communication system of the LTE system and has improved data transmission capability in comparison with the LTE.
In general, the LTE means a base station and a terminal equipment corresponding to a release 8 or a release 9 of 3GPP standardization group, and the LTE-A means a base station and a terminal equipment corresponding to a release 10 of the 3GPP standardization group. The 3GPP standard group has been progressing standardization with respect to a follow-up release based on the LTE-A system and having improved performance after standardization of the LTE-A system.
The existing 3rd Generation (3G) and 4th Generation (4G) wireless packet data communication systems such as HSDPA, HSUPA, HRPD, and LTE/LTE-A employ an Adaptive Modulation and Coding (AMC) scheme, a channel-sensitive scheduling scheme, and the like to improve transmission efficiency. With the use of the AMC scheme, a transmitter can adjust the amount of transmission data according to the channel state. That is, when the channel state is not ‘Good’, the transmitter reduces the amount of transmission data to adjust the reception error rate to a desired level, and when the channel state is ‘Good’, the transmitter increases the amount of transmission data to adjust the reception error rate to the desired level and to efficiently transmit a large volume of information. With the use of the channel-sensitive scheduling-based resource management method, the transmitter selectively provides a service to a user having a good channel state among a plurality of users, thus increasing the system capacity compared to the method of assigning a channel to one user and providing a service to the user with the assigned channel. Such a capacity increase as in the above description is referred to as “multi-user diversity gain”. In summary, the AMC method and the channel-sensitive scheduling method each are a method of applying the appropriate modulation and coding techniques at the most-efficient time determined depending on the partial channel state information fed back from a receiver.
The AMC scheme, when used together with a Multiple Input Multiple Output (MIMO) transmission scheme, may include a function of determining the rank or the number of spatial layers of a transmission signal. With regard to this, the AMC scheme determines an optimal data rate in consideration of not only a coding rate and a modulation scheme, but also the number of layers for transmission using MIMO.
Recently, intensive research is being conducted to replace Code Division Multiple Access (CDMA), the multiple access scheme used in the 2nd and 3rd generation mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation system. The 3GPP and 3GPP2 have started their standardizations on the evolved systems employing the OFDMA. The OFDMA scheme may have a capacity increase compared to the CDMA scheme. One of several factors for the capacity increase in the OFDMA scheme is the capability to perform scheduling on the frequency axis (frequency domain scheduling). Although a capacity gain is acquired according to the time-varying channel characteristic using the channel-sensitive scheduling method, it is possible to obtain a higher capacity gain with use of the frequency-varying channel characteristic.
FIG. 1 illustrates a time-frequency resource in an LTE/LTE-A system.
In FIG. 1, a wireless resource transmitted from an eNodeB (eNB) to a terminal is divided into a Resource Block (RB) 101 unit on a frequency axis, and is divided into a subframe 102 unit on a time axis. In the LTE/LTE-A system, the RB generally includes 12 subcarriers and occupies 180 kHz of bandwidth, and the subframe includes 14 OFDM symbol periods and occupies 1 msec of a time period. The LTE/LTE-A system may assign a resource in a subframe unit on the time axis and assign a resource in an RB unit on the frequency axis in performing scheduling.
FIG. 2 illustrates a wireless resource of one subframe and one RB, which is a minimum unit schedulable in a downlink in the LTE/LTE-A system.
The wireless resource shown in FIG. 2 includes one subframe on the time axis and includes one RB on the frequency axis. Such a wireless resource includes 12 subcarriers in a frequency region, includes 14 OFDM symbols in a time region, and thus includes 168 inherent frequency and time position. In the LTE/LTE-A system, each inherent frequency and time positions of FIG. 2 is referred to as a Resource Element (RE).
The following several different types of signals may be transmitted in the wireless resource shown in FIG. 2.
1. Cell specific Reference Signal (CRS): The CRS is a reference signal periodically transmitted for all terminals included in one cell, and may be used by a plurality of terminals.
2. DeModulation Reference Signal (DMRS): The DMRS is a reference signal transmitted for a specific terminal, and is transmitted only when data is transmitted to a corresponding terminal. The DMRS may be configured by a total of 8 DMRS ports. In the LTE/LTE-A system, ports from port 7 to port 14 correspond to DMRS ports and ports maintain the orthogonality in order to prevent generation of interference between them by using a CDM or a FDM.
3. Physical Downlink Shared CHannel (PDSCH): The PDSCH is a data channel transmitted in a downlink, is used by the base station to transmit traffic to the terminal, and is transmitted by using an RE where a reference signal (the CRS or the DMRS) is not transmitted in the data region of FIG. 2.
4. Channel Status Information Reference Signal (CSI-RS): The CSI-RS is used in measuring a channel state of the reference signal for transmitting terminals included in one cell. A plurality of CSI-RSs may be transmitted from one cell.
5. Other control channels (PHICH, PCFICH and PDCCH): The PHICH, PCFICH and PDCCH are used in providing control information required to receive a Physical Downlink Shared CHannel (PDSCH) or in transmitting an ACK/NACK for operating an HARQ with respect to a transmission of an uplink data.
In addition to transmission of the signals enumerated above, the LTE-A system allows configuration of muting, by which a CSI-RS transmitted from another base station can be received without interference by UEs of a corresponding cell. The muting may be applied to a position where the CSI-RS can be transmitted, and in general, the terminal receives a traffic signal by skipping a corresponding wireless resource. In the LTE-A system, the muting is also referred to as zero-power CSI-RS, and this is because the muting is applied to a position of the CSI-RS and a transmission power is not transmitted in a corresponding position due to a characteristic of the muting.
In FIG. 2, the CSI-RS may be transmitted by using a part of the positions (patterns) marked by A, B, C, D, E, F, G, H, I, and J according to the number of antennas for transmission of the CSI-RS. Further, the muting may be also applied to a part of the positions marked by A, B, C, D, E, F, G, H, I, and J. In special, the CSI-RS may be transmitted with two, four or eight REs depending on the number of antenna ports transmitting the CSI-RS. In FIG. 2, when the number of the antenna ports is two, the CSI-RS is transmitted from a half of one pattern, when the number of the antenna ports is four, the CSI-R is transmitted from a whole of the one pattern, and when the number of the antenna ports is eight, the CSI-RS is transmitted by using two patterns. In contrast, in a case of the muting, the CSI-RS is always transmitted by using one pattern unit. That is, the muting may be applied to a plurality of patterns, but cannot be applied to only a part of one pattern when a muting position does not overlap a CSI-RS position. But, the muting may be applied to the partial of one pattern only when the muting position overlaps the CSI-RS position.
Meanwhile, in a cellular system, the reference signal should be transmitted in order to measure a downlink channel state. In the case of the LTE-A system of the 3GPP, the terminal measures a channel state between the base station and the terminal by using a CSI-RS transmitted by the base station. Basically, several factors should be considered in the channel state, here, the downlink interference is included. The downlink interference includes interference signals, thermal noise, and the like caused by antennas belonging to a neighbor base station, and is an important factor when the terminal determines a downlink channel condition. For example, when a base station which has one transmission antenna transmits the reference signal to a terminal which has one reception antenna, the terminal should determine a proportion of symbol energy to interference (Es/Io) by determining symbol-specific energy receivable with a downlink in a reference signal received from the base station and interference to be received simultaneously in a period in which a corresponding symbol is received. The determined Es/Io is informed of to the base station to allow the base station to determine a transmission speed in transmitting data to the terminal with the downlink.
In a case of a general mobile communication system, a base station equipment is disposed in a central point of each cell, and a corresponding base station equipment communicates with a terminal by using one or more antennas positioned in a limited place. As described above, a mobile communication system in which antennas included in one cell are disposed at a same position is called a Centralized Antenna System (CAS). In contrast, a mobile communication system in which antennas (Remote Radio Heads; RRHs) belonging to one cell are located at distributed positions in the cell is called a Distributed Antenna System (DAS).
FIG. 3 illustrates a signal transmitted according to a time in the LTE/LTE-A system.
In FIG. 3, one radio frame corresponding to 10 msec is transmitted. In the LTE/LTE-A system, one radio frame includes ten subframes. In addition, the subframes forming one radio frame are configured with a normal subframe or a Multimedia Broadcast multicast service Single Frequency Network (MBSFN) subframe. With regard to a difference between the normal subframe and the MBSFN subframe, in a case of the normal subframe, the CRS is included in the data region. In contrast, in a case of the MBSFN subframe, the CRS is not included in the data region. In the LTE/LTE-A system, in the case of the MBSFN subframe, since only terminals after a release 10 receive data by using the DMRS, it is not necessary for the CRS to receive the data in the data region. In contrast, in the case of the normal subframe, since not only the terminals after the release 10 but also terminals of a release 8 and a release 9 may receive the data, the CRS needed in receiving the data by the terminals is transmitted. Which subframe is the normal subframe among the subframes in one radio frame and which subframe is the MBSFN subframe among the subframes in one radio frame are informed of from the base station to the terminal by using a higher layer signaling.
FIG. 4 illustrates an example of a distributed disposition of antennas in a normal distributed antenna system.
FIG. 4 corresponds to a distributed antenna system including two cells 400 and 410. The cell 400 includes one high-output antenna 420 and four low-output distributed antennas 440 to 443. The 410 is formed identically to the cell 400. The high-output antenna 420 provides a minimum service to the whole area included in a cell area. In contrast, the low-output distributed antennas 440 to 443 may provide a service based on a fast data speed to limited terminals in a limited area of the cell. In addition, all of the low-output distributed antennas 440 to 443 and the high-output antenna 420 are connected to a central controller (not shown) with 430, and operated according to a scheduling and a wireless resource assignment of the central controller. In the distributed antenna system, one or more antennas may be disposed at a position of one geographically separated antenna. In the present invention, one or more antennas disposed at the same position in the distributed antenna system are referred to as an antenna group (RRH group). In the distributed antenna system shown in FIG. 4, the terminal receives a signal from one geographically separated antenna group. Signals received from other antenna groups affect the terminal as interference.
When the CRS is transmitted in the distributed antenna system as shown in FIG. 4, all antennas included in one cell participate in transmitting the CRS. All antennas included in one cell transmit the CRS, or do not transmit another signal at an RE position where the CRS is transmitted so as not to generate interference in transmitting the CRS from other antennas. That is, when all antennas included in one cell transmit a signal for the CRS, a data signal transmitted based on the CRS is transmitted from all antennas included in a cell. In contrast, when only partial antennas included in one cell transmit the signal for the CRS, an antenna which does not transmit the signal for the CRS does not transmit another signal at the RE position where a corresponding CRS is transmitted so as not to generate interference to the CRS transmitted from other antennas. In addition, the data signal transmitted based on the CRS is transmitted from only antennas transmitting the CRS.
FIG. 5 illustrates transmitting the data signal based on the CRS and transmitting the data signal based on the DMRS in the distributed antenna system.
Referring to FIG. 5, the data is transmitted based on the CRS in a cell 500, and the data is transmitted based on the DMRS in a cell 510. In the data transmission base on the CRS, all antennas included in a cell transmit a signal for a specific terminal. In contrast, in the data transmission based on the DMRS, a portion of the antennas included in the cell transmits a data signal for the specific terminal, but other antennas which do not transmit the data signal for the specific terminal may transmit the data signal to another terminal. In the cell 500, all antennas included in the cell transmit a signal for a UE1. In cell 510, the DMRS is assigned to each of UE3 and UE4 by using two different antennas, and therefore UE3 and the UE4 may receive data.
In the distributed antenna system as shown in FIG. 5, the transmission based on the CRS has advantages and disadvantages as follows compared with the transmission based on the DMRS. When the data is transmitted based on the CRS, since a signal may be transmitted from all antennas of the distributed antenna system, a signal received by the terminal has a comparatively superior Signal to Interference and Noise Ratio (SINR), and as a result, superior reception performance may be obtained. However, since the CRS always exists in the normal subframe, when the data is transmitted by using the DMRS in the normal subframe, a wireless resource to be additionally assigned for the DMRS is required, and therefore, a wireless resource for the data transmission comparatively becomes lower. In addition, the CRS is a common signal usable by all terminals included in the cell, and therefore, different CRSs may not be assigned to each terminal. That is, when a signal is transmitted to a specific terminal by using the CRS, all antennas included in the cell should transmit a signal for this terminal or should not generate another signal. This may incur a problem of assigning an unnecessary wireless resource in the data transmission for the CRS, with regard to a wireless management of the distributed antenna system.
In the LTE/LTE-A system, the base station may set a unique transmission mode to each terminal. Here, the base station selects a transmission mode capable of providing an optimum performance to each terminal in consideration of a channel condition of the terminal, a function implemented to the base station, etc. For example, a transmission mode 9 supported in an LTE/LTE-A release 10 performs a downlink transmission based on the DMRS and may transmit data from a maximum of eight transmission antennas. Separately from such a transmission mode, the LTE/LTE-A system also supports a fallback transmission. The fallback transmission is for transmitting data to a terminal of which a channel condition is not good. For example, when a downlink transmission method according to a transmission mode is not proper for the channel condition of the terminal, the base station changes the transmission mode of the terminal to a more proper mode by using the fallback transmission.
In the fallback transmission, receiving data stably is importance, and therefore, in general, a transmit diversity capable of properly coping with a dynamic change of a wireless channel is utilized. In the normal subframe in which the CRS is included in LTE/LTE-A release 10, the fallback transmission is performed by utilizing a Space Frequency Block Code (SFBC) which is a kind of transmission diversity method. But, in the MBSFN subframe in which the CRS is not included, the fallback transmission is performed by using a DMRS port 7.
Table 1 below is a summary of a transmission mode and a fallback transmission which may be set to a release 10 terminal in the LTE/LTE-A.
TABLE 1DownlinkTransmission modeFallbacktransmission9transmissionNormal SubframeDownlinkSFBC downlinktransmission usingtransmission basedranks 1 to 8 basedon the CRSon DMRSMBSFN SubframeDownlinkDownlinktransmission usingtransmission usingranks 1 to 8 basedport 7 basedon DMRSon the DMRS
The transmission mode of the LTE/LTE-A terminal is set according to a determination of the base station. But, the fallback transmission is not set by the base station, and is always performed as noted table 1 above.
In the LTE/LTE-A system, when the fallback is transmitted based on the DMRS, the DMRS is scrambled for interference randomization. A sequence for the DMRS scrambling may be different depending on which initial state is used in a sequence generator, and the initial state is defined as noted in Equation 1 below.cinit=(└ns/2┘+1)·(2NIDcell+1)·216+nSCID  Equation 1
In Equation 1 above, ns is a slot ID indicating nth slot in the radio frame. In a case of the LTE/LTE-A system, one subframe includes two slots. In addition, 2NIDcell is a cell ID included in each cell, has values of 0 to 503, and is information obtained by receiving the CRS of a corresponding cell when the terminal accesses to the cell initially or performs a handover. nSCID nSCID is an ID of the scrambling sequence, and is fixed as zero in a case of the DMRS for the fallback transmission.
In the LTE/LTE-A system, when the fallback transmission is performed to the terminal, the base station transfers control information by using a Physical Downlink Control Channel in order to inform of fallback transmission to the terminal. The control information transmitted with the PDCCH for the fallback transmission is transmitted in a form according to a Downlink Control Information (DCI) format 1A. The PDCCH is a channel transmitted based on the CRS.